1. Field of the Invention
The invention relates to catalyst means for the catalytic treatment of exhaust gas. Such catalyst means intended for installation in a housing of a catalytic converter are frequently also referred to as the substrate of the catalytic converter.
The catalyst means have passages for the exhaust gas and are intended in particular for purifying and/or detoxifying exhaust gas produced by a petrol combustion engine or possibly by other internal combustion engine and flowing through the passages by a catalytic treatment, i.e. for freeing said exhaust gas from pollutants, by converting the latter chemical reaction. The internal combustion engine may belong, for example, to an automobile or to another motor vehicle or serve for stationary operation--for example for driving an emergency power generator.
2. Description of the Prior Art
Known catalyst means or substrates have a packet of sheet metal members which rest against one another at certain points and together bound passages. Said sheet metal members are provided with coatings which contain alumina and catalytically active material which is applied to the alumina and consists of at least one noble metal. The sheet metal members are held together by retaining means.
European Patent Disclosure 0 514 326 discloses inter alia, for exude, catalyst means having disk-like sheet metal members. The successive sheet metal members have intersecting waves, are arranged between flat end members and are held together by these and by bolts which pass through them, one of the end members being formed by a housing wall.
When a catalytic converter is installed in an automobile, vibrations, accelerations and hard, abrupt impacts acting on the catalyst means during the use of the automobile are produced. In addition, the catalyst means are heated by the exhaust gas during operation and then cooled again, local temperature differences being produced and the dimensions of the various parts changing as a function of the local temperature. In known catalyst means of the type described, the vibrations, accelerations, impacts and thermally produced dimensional changes result in the sheet metal members moving along the bolts and the compressive forces exerted by the sheet metal members and end members on one another changing as a function of time and place. Very large and, in some cases, abrupt changing compressive forces may occur temporarily from place to place and cause the alumina and the catalytically active material to split off.
In order to obtain a large number of passages per unit area and to keep the weight, the heat capacity and the material costs low, it is desirable to produce the catalyst means from sheet metal members which are as thin as possible. However, during use of a catalytic converter, forces which are also directed transversely to the bolts are exerted at the edges of the holes in the sheet metal members through which the bolts pass and may damage the stated edges, in particular in the case of thin sheet metal members. In addition, movements of the sheet metal members in a direction transverse to the bolts may occur and may rub off catalytically active material.
A catalytic converter disclosed in French Patent Disclosure 2 617 903 has a housing which contains catalyst means having a packet of annular sheet metal members which have coatings containing catalytically active material. The sheet metal members are in general conical, but at least one sheet metal member of each pair of adjacent sheet metal members is provided with waves or bulges. The sheet metal men, bets are arranged in a cage which has an annular flange at one end of the packet, a plate at the other end and some rods connected to said plate and to the annular flange and distributed around the packet. According to the last-cited publication, the sheet metal members may be connected to one another or to the annular flange or to the plate by spot welding. Since the sheet metal members touch one another at least partly only in the case of linear wave summits or point-like bulge peaks and have coatings, it would however be very difficult and expensive to weld all sheet metal members to one another in pairs. Moreover, in spite of such spot welds from point to point, which are practically inevitably staggered relative to one another in the axial direction of view, the sheet metal members could still experience relatively strong deformation and movement relative to one another. Since the sheet metal members are subjected to vibrations, other accelerations, temperature change and temperature gradients during the use of the catalytic converter, there is a great danger in this catalytic converter too that the sheet metal members and especially their coatings will be damaged.
International Patent Disclosure 93/21 431 furthermore discloses catalyst means having a cylindrical winding and passages parallel to its axis. The winding is formed from two pairs of adjacent strips, a strip having waves parallel to the axis and a waveless, originally flat strip alternating with one another. The strips originally consist of aluminum foils and are then converted by anodic oxidation into alumina and coated with catalytically active material. The catalyst means have two strip-like, electrically heatable heating elements, each of which is arranged between two adjacent strips of the winding. The two heating elements are present on one and the same lateral surface of the winding, at one end of the passages. The heating elements are preferably provided with coatings of alumina and catalytically active material.
When an internal combustion engine supplying exhaust gas to the catalyst means is started, the heating elements serve to heat the exhaust gas flowing into the passages of the catalyst means and to heat the catalyst means. Since the heating elements are at right angles to the waves and passages, they touch the strip adjacent to them and having waves at most in very small, more or less linear sections of the wave summits and are scarcely firmly connected to the strips. Since the strips of the prepared catalyst means consist mainly of alumina and since the heating elements preferably have coatings of alumina, it would in particular also be practically impossible to connect the strips of the winding and the strip-like heating elements to one another by welding or hard soldering. Since furthermore heating elements are present only on one lateral surface of the winding, they scarcely contribute towards holding together the strips of the winding and connecting them to one another. The last-cited publication also discloses no other retaining means for holding together the winding. It therefore appears probable that the strips of the winding can move relative to one another and may damage one another if the winding is subjected to vibrations, other accelerations, temperature gradients and temperature changes during use.